Keyframing problem. Adds keyframes on all objects instead of the one that I selected
As has been mentioned in the other answer, the animations of the separate objects are still linked. 

You can see this if you go to the 'Dopesheet' and change the view mode to 'Action Editor'. The action datablock, which stores the animation, has '13' next to it showing that all 13 characters of text are using this shared animation:



To make all the objects animations separate you can select all the objects in the 3D view and do U> Object Animation. Now you can edit all the animations separately.

You could also do this individually by clicking on the '13' next to the action name, which creates a copy of the animation data, but you would have to do this on each object.